There are several well known techniques for receiving amplitude modulated signals, and in particular, signals modulated by amplitude shift keying a carrier signal. In such AM signals, variations in the magnitude represent digital data, the digital data of which may represent information to be transmitted. Amplitude modulation of digital data is used in a variety of applications including, but not limited to: radio watches, radio frequency identification (RFID) tags, fiber optics, and cable modems.
A first well known technique for receiving amplitude shift keyed modulated signals includes directly converting the AM signal to a baseband signal. FIG. 1 shows a conventional direct conversion receiver 50 suitable for AM demodulation. Antenna 5 is configured to receive a low frequency broadcast radio signal. For radio watch applications, the frequency of the radio signal is typically between 40 kHz to 100 kHz. Amplifier 10 is configured to receive an output from the antenna and generate a modulated signal 15. Modulated signal 15 is then mixed with a first demodulating signal 78 by mixer 20, low pass filtered by filter 35, and amplified by amplifier 67 to produce a baseband signal. This baseband signal can thereafter be converted to a digital signal representative (less errors) of the digital data that was intended to be received.
The first demodulating signal 78 that is mixed with the modulated signal 15 by mixer 20 can be produced by a frequency synthesizer 73 and generator 77. In one example, the modulated signal 15, in addition to being mixed with the first demodulating signal 78 by mixer 20, is additionally mixed with a second demodulating signal 79 by mixer 40. The output of mixer 40 is a phase error signal 75 that is received by a frequency synthesizer 73. The frequency synthesizer 73 also receives a frequency control signal 97 and provides a local carrier signal 76 to a generator 77. In one example, the generator 77 is an I/Q generator and produces an in-phase output and a quadrature phase output. In this example, the in-phase output signal is the first demodulating signal 78 provided to mixer 20 and the quadrature phase output signal is the second demodulating signal 79 provided to mixer 40. The conventional direct conversion receiver 50 of FIG. 1 is most often used in a low noise environment, and may provide better sensitivity to weak broadcast radio signals as compared to other receiver topologies. However, in high noise environments, it may be advantageous to use a more complex approach.
FIG. 2 shows a conventional image-rejection heterodyne receiver 100 suitable for AM demodulation in high noise environments. A low frequency broadcast radio signal is received by antenna 105 and subsequently amplified by amplifier 110 to produce a modulated signal 115. The modulated signal 115 is applied as an input to mixers 120 and mixer 140, for mixing the signal with the first and second demodulating signals 178 and 179, respectively. The output of mixer 120 is applied to phase shifter 130 in series with low pass filter 135. Similarly, the output of mixer 140 is applied to phase shifter 150 in series with low pass filter 155. The output of filter 135 and the output of filter 155 are input to a summer 163, which adds the two signals together and outputs the sum to amplifier 167. The output of amplifier 167 is an intermediate frequency signal suitable for subsequent processing.
The first and second demodulating signals 178 and 179 of the conventional image-rejection heterodyne receiver 100 can be produced in a manner similar to that as shown in FIG. 1. However, the frequency synthesizer 173 is configured to receive a reference timing signal 174 (which can be generated by an external source such as a crystal oscillator) instead of the phase error signal 75 (as shown in the conventional direct conversion receiver of FIG. 1). Frequency synthesizer 173 also receives a frequency control signal 197 and provides a local carrier signal 176 to a generator 177. As above, generator 177 can be an I/Q generator and produce an in-phase signal (the first demodulating signal 178) and a quadrature phase signal (the second demodulating signal 179).
In general, portable radio devices must be designed to properly operate in both low noise and high noise environments. In addition, many such devices (including radio wrist watches) have significant power and size design constraints. A radio watch implementing a conventional direct conversion receiver may operate at lower power and have a smaller footprint than a conventional image-rejection heterodyne receiver. However, the device's performance will suffer in a high noise environment. Those radio watches implementing a conventional image-rejection heterodyne receiver will have better performance in a high noise environment, but may be larger than would be desired in some commercial applications. Furthermore, when operating in a low noise environment, a radio watch that includes a conventional image-rejection heterodyne receiver may also consume more power than desired.
Therefore, a need exists for an AM receiver that can combine advantageous properties of both direct conversion and image-rejection heterodyne architectures.